Pushing and pulling get the cell moving.

Borisy deciphered the motor’s design by watching its creation. The ignition switch was a gentle puff of fluid on some stationary fragments to coax them into movement.

Actin and myosin in a moving cell.

The push squished together enough myosin to start a contraction, thus lifting what was now the ‘back’ of the cell. The cell then acted like a plastic paddling pool, in which a push on one side makes water slop into the bulge on the other side. The only actin that could grow into the new space on the other side was the actin that was already pointing forwards, so this actin pushed the cell edge further outwards and forwards. The changes were self-perpetuating – actin pushing at the front made the cell rear bunch up; the resulting myosin contraction pulled up the back, making more room for actin extension at the front – and the cell fragment sped off.

Myosin at work.

The same two-part motor seems to work in whole fish cells. Borisy found that strong actin cables push out the very front of the cells, but, further back from the front, the cables weaken and can be deformed by groups of myosin motors.

The myosins run along multiple splayed actin cables at once and act like a zipper, collapsing the loose mesh into a tight bundle. The mesh is anchored to the surface below the front of the cell, so it collapses forwards, and connecting cables running back to the nucleus pull everything else forward with it.

Borisy’s model predicts that a moving cell will exert force near where the mesh is collapsing. Sure enough, when Michael Sheetz of Duke University watches cells move over a series of tiny cantilevers, the cells exert the greatest forces on the cantilevers that are underneath the collapsing mesh.

Making a Movement Machine